Integrated circuits are frequently used in the presence of radiation such as x-rays, gamma-rays, photons, particles, etc. A radiation strike can deposit charge in silicon and, therefore, can cause upsets in the integrated circuits. The most common upset causes are from such particles as protons, neutrons, and heavy ions. As a result of such radiation, charges can be collected at circuit nodes that send the nodes to unintended opposite voltage states (e.g., from high to low). When this voltage state change happens to a data storage circuit, for example, the data storage nodes change to the wrong data states.
All circuits can tolerate some amount of deposited charge that does not cause a node to change states. However, all circuits also have some deposited charge threshold above which the node state will be changed. This threshold is referred to as the critical charge (i.e., Qcrit) for upset. Such node state changes are defined as radiation induced upsets. When radiation particles, which are particles that are discrete in time and space, cause a data upset, the data upset is referred to as a single event upset (SEU).
Various arrangements have been provided to increase the immunity of integrated circuits from single even upsets. For example, co-pending U.S. application Ser. No. 10/034,808 filed on Dec. 28, 2001 gives several examples of SEU hardening techniques for preventing unintended data state changes in storage elements in response to radiation strikes.
FIG. 1 shows another technique to increase the immunity of integrated circuits from single event upsets. As shown in FIG. 1, an integrated circuit is provided with triple redundancy as indicated by instantiations 10, 12, and 14 of the same integrated circuit. The instantiation 10 of this integrated circuit is coupled to an input A of a majority voter circuit 16, the instantiation 12 of the same integrated circuit is coupled to an input B of the majority voter circuit 16, and the instantiation 14 of the same integrated circuit is coupled to an input C of the majority voter circuit 16.
The majority voter circuit 16 provides an output on an output line 18 based on a majority vote between the inputs A, B, and C. For example, if the inputs A and B are the same but are different from the input C, then the output on the output line 18 is based on the inputs A and B. Alternatively, if the inputs B and C are the same but are different from the input A, then the output on the output line 18 is based on the inputs B and C. However, if the inputs A and C are the same but are different from the input B, then the output on the output line 18 is based on the inputs A and C.
The majority voter circuit 16 comprises a first inverter having a p-channel transistor 20 and an n-channel transistor 22 coupled in series between VDD and ground. The gate of the p-channel transistor 20 and the gate of the n-channel transistor 22 are coupled to the input A, and the junction between the p-channel transistor 20 and the n-channel transistor 22 is coupled to the output line 18.
The majority voter circuit 16 also comprises a second inverter having a p-channel transistor 24 and an n-channel transistor 26 coupled in series between VDD and ground. The gate of the p-channel transistor 24 and the gate of the n-channel transistor 26 are coupled to the input B, and the junction between the p-channel transistor 24 and the n-channel transistor 26 is coupled to the output line 18.
The majority voter circuit 16 further comprises a third inverter having a p-channel transistor 28 and an n-channel transistor 30 coupled in series between VDD and ground. The gate of the p-channel transistor 28 and the gate of the n-channel transistor 30 are coupled to the input C, and the junction between the p-channel transistor 28 and the n-channel transistor 30 is coupled to the output line 18.
Accordingly, radiation may strike the sensitive area of one of the instantiations 10, 12, and 14 of the integrated circuit causing the output of that instantiation to assume an incorrect output state radiation. However, it is not likely that radiation will simultaneously strike the sensitive area of a second of the instantiations 10, 12, and 14 of the integrated circuit causing the output of this second instantiation to assume the same incorrect output state. Because it is not likely that radiation will strike the sensitive areas of two or more of the instantiations 10, 12, and 14 at the same time, the output on the output line 18 will be in the correct state because the majority voter circuit 16 will vote on a majority basis to select the inputs unaffected by the radiation and thus control the output on the output line 18 at the intended output state.
As an example, an SEU event may occur in a sensitive area of the instantiation 10 that causes the input A to transition from a low state to a high state so as to turn on the n-channel transistor 22. However, as long as the sum of the drain currents in the p-channel transistors 24 and 28 is greater than the drain current of the n-channel transistor 22, the output signal on the output line 18 will not change states.
Unfortunately, it is distinctly possible that the drain currents in the p-channel transistors 24 and 28 will not be greater than the drain current of the n-channel transistor 22, particularly under worst case conditions, in which case the circuit of FIG. 1 is not hardened against to SEU events. Also, even if the total drain current of the p-channel transistors 24 and 28 is larger than the drain current of the n-channel transistor 22, the speed of the majority voter circuit 16 of FIG. 1 is adversely affected in a significant way due to the competition between the drain currents in the p-channel transistors 24 and 28 and the n-channel transistor 22. Moreover, if a strong SEU event occurs in one of the transistors 20-30, the affected transistor can turn on so hard that its drain current will overcome the drain currents of the other transistors and produce an erroneous output signal on the output line 18. Thus, the majority voter circuit 16 itself is not SEU hardened.
The present invention is directed to a majority voter circuit that overcomes one or more of these or other problems.